get/import/url: handle existing output directory

[efiop]

Currently if you run

dvc get https://github.com/iterative/example-get-started model.pkl --out .

it will nuke your cwd. To fix that we need to properly handle existing
directories, same as usual unix cli utilities do that.

• Have you followed the guidelines in our
  Contributing document?

• Does your PR affect documented changes or does it add new functionality
  that should be documented? If yes, have you created a PR for
  dvc.org documenting it or at
  least opened an issue for it? If so, please add a link to it.

iterative/dvc.org#699

---

[shcheklein]

@efiop @jorgeorpinel should we create a ticker/clarify the --out behavior in docs?

[efiop]

@shcheklein Will do, I didn't click the docs checkbox in the description for that reason :)

[efiop]

@shcheklein The docs are written correctly already, it was a bug on our side.

[shcheklein]

@efiop I'm not sure that it's clear from this ... specify a path (directory and file name) that it one can omit the filename. Also, it's no about files only, it can be a dir name. cc @jorgeorpinel

[efiop]

@shcheklein I was more focused on The default value (when this option isn't used) is the current working directory (.) and original file name. But ok, I'll send a PR soon.

[efiop]

@shcheklein iterative/dvc.org#699

[jorgeorpinel]

I was more focused on The default value (when this option isn't used) is the current working directory (.) and original file name....

@efiop is that no longer the case? I don't see that text changed in the PR. Thanks

[efiop]

@jorgeorpinel The defaults are the same, so I didn't change that text.